On Fri, Dec 16, 2016 at 6:54 AM, Will Deacon [off-list ref] wrote:

Hi Rob,

On Tue, Dec 06, 2016 at 06:30:21PM -0500, Rob Clark wrote:

quoted

On Thu, Aug 18, 2016 at 9:05 AM, Will Deacon [off-list ref] wrote:

quoted

Enabling stalling faults can result in hardware deadlock on poorly
designed systems, particularly those with a PCI root complex upstream of
the SMMU.

Although it's not really Linux's job to save hardware integrators from
their own misfortune, it *is* our job to stop userspace (e.g. VFIO
clients) from hosing the system for everybody else, even if they might
already be required to have elevated privileges.

Given that the fault handling code currently executes entirely in IRQ
context, there is nothing that can sensibly be done to recover from
things like page faults anyway, so let's rip this code out for now and
avoid the potential for deadlock.

so, I'd like to re-introduce this feature, I *guess* as some sort of
opt-in quirk (ie. disabled by default unless something in DT tells you
otherwise??  But I'm open to suggestions.  I'm not entirely sure what
hw was having problems due to this feature.)

On newer snapdragon devices we are using arm-smmu for the GPU, and
halting the GPU so the driver's fault handler can dump some GPU state
on faults is enormously helpful for debugging and tracking down where
in the gpu cmdstream the fault was triggered.  In addition, we will
eventually want the ability to update pagetables from fault handler
and resuming the faulting transition.

I'm not against reintroducing this, but it would certainly need to be
opt-in, as you suggest. If we want to try to use stall faults to enable
demand paging for DMA, then that means running core mm code to resolve
the fault and we can't do that in irq context. Consequently, we have to
hand this off to a thread, which means the hardware must allow the SS
bit to remain set without immediately reasserting the interrupt line.
Furthermore, we can't handle multiple faults on a context-bank, so we'd
need to restrict ourselves to one device (i.e. faulting stream) per
domain (CB).

I think that means we want both specific compatible strings to identify
the SS bit behaviour, but also a way to opt-in for the stall model as a
separate property to indicate that the SoC integration can support this
without e.g. deadlocking.

How do you feel about a short-term step to keep things in irq context,
but enable stall mode?  I'm debugging an issue, where it appears that
the gpu cannot handle a non-stalled fault (triggers some fairly
bizarre follow-on problems).  So I think even if we don't add a fault
handler callback, we still want to set the CFCFG bit and
RESUME_TERMINATE in the fault handler on this hardware.

BR,
-R